Biological cells, such as cells taken from the human body, can be investigated by treating the cells with various chemical substances and observing and measuring luminous phenomena (e.g., fluorescence of the cells) occurring after this treatment and irradiation of the cells by light. By way of example, cells are stained with two different fluorescent dyes, one of which causes phenomena of fluorescence characteristic of the DNA and the other one characteristic of the protein of the cell.
To accomplish this, dyes are employed which show peaks, or maxima, of absorption at certain wavelengths during the fluorescence excitation. When using devices available prior to the present invention, errors occur in the case of simultaneous measurement, because the fluorescence spectra of the dyes utilized are comparatively broad and overlap one another. In these instruments part of the light of one dye invariably reaches the photomultiplier intended for the other dye. A device built for this purpose is described by M. Stohr in "Double Beam Application in Flow Technic and Recent Results", Pulse-Cytophotometry, 1976, pp. 39-45. The apparatus is equipped with an argon-ion laser and a helium-cadmium laser emitting at wavelengths of 448 nm. and 441 nm., respectively.
It also has special mirrors for 325 nm. According to Stohr, difficulties can be avoided if the points of interaction between a particle and the illuminating laser beams are physically separated. However, no solution is offered to the problem in that paper.
Other examples of prior art devices in this general field are U.S. Pat. Nos. 3,513,319; 3,541,336 and 3,609,048.